Method of coating tin over basis metals



United States Patent 3,323,938 METHOD OF COATING TIN OVER BASIS METALS Leonard M. Vaught, Lake .iackson, Tex., assignor to The Dow Chemical Company, Midland, Mich, a corporation of Delaware N Drawing. Filed Nov. 18, 1963, Ser. No. 324,231

' 7 Claims. (Cl. 117-71) This invention relates to a method of coating tin over basis metals, and more particularly to a method of coating tin over lead or zinc.

It is desirable in plating metals that the plate be bright, uniform and free of pin holes. Further, it is desirable that the rate of coating or plating be relatively rapid. Previously known methods of coating tin over basis metals such as suggested in Metal Finishing Guidebook, generally result in thin coatings which are dull in the unpolished state, are porous, and are accumulated on the base metal at a slow rate.

I have discovered a method whereby tin may be coated over basis metals to produce a tight, lustrous, substantially pin-hole free surface at a rate which is substantially more rapid than the previously known methods.

In accordance with my invention, tin may be plated or coated over basis metal by contact with an aqueous bath containing from about 6 percent by weight to the saturation limit of tin salt, and preferably from about 10 percent by weight to the saturation limit of the salt in the aqueous solution. When concentrations of tin salt in the 6 to 10 weight percent range are employed, it is generally advantageous to have present at least about percent by weight ZnCl Plating or coating in accordance with the present invention takes place at a temperature of from about 200 to about 250 degrees centigrade, while a plating or coating temperature range of from about 230 to about 0 degrees centigrade is preferred. Coating at the desired temperature may be accomplished in a variety of ways. The solution may be held at the desired coating temperature and room temperature basis metal subsequently immersed therein; the bath may be maintained at room temperature and the basis metal preheated to the desired coating temperature range prior to immersion, or both the bath and the metal may be heated to the desired coating temperature range prior to immersion. The present invention is not limited to immersion coating. Any other suitable method of contacting the solution with the basis metal at the desired temperature, such as by spraying solution onto the basis metal, may be employed.

Coating or plating in accordance with the present invention generally takes place in a short time, two seconds at the appropriate temperature being an effective minimum time. When a hot solution and cool basis metal, or cool solution and hot basis material are employed, temperatures at the surface being plated are generally transitory. What is required, for purposes of the present invention, is that for a period of at least two seconds the surface of the basis metal, or the solution, or both are within the temperature range of from about 200 to about 250 degrees centigrade.

Basis metals contemplated for use in the present invention are lead and zinc. The basis metal may be a mixture or alloy containing major proportions (at least 50 percent by weight) of lead or zinc, or may be another material having a plate or coating of lead, zinc, or mixtures or alloy containing major proportions thereof.

Suitable tin salts for use in the aqueous coating solutions of the present invention are those which are soluble in water and relatively stable at the temperatures of coating (e.g., ZOO-250 C.). Non-limiting examples of typical 3,323,938 Patented June 6, 1967 "ice salts are tin chloride, tin fluoride, tin iodide, tin bromide, tin tartarate, tin sulfate, tin chromate, and the like.

The following examples are set forth to illustrate, and are not to be construed to limit, the present invention.

Example ].-A chemically pure molten zinc bath Was used to hand dip a galvanized coating from 2 to 2 mils thick on 16 gauge steel. After the galvanizing treatment, the metal, at a temperature of from about 455 degrees centigrade, was immediately quenched in a room temperature aqueous solution containing about 17 percent by weight stannous chloride. After the metal reached the quench bath temperature (passing through the temperature range ZOO-250 C. in approximately 2 seconds) it was removed and washed with water. Observation of the metal revealed a bright, tight, tin coating that did not rapidly react with hydrochloric acid.

Example 2.-Employing the molten zinc galvanizing bath described in Example 1, coupons of 16 gauge steel were galvanized by hand dipping. One such coupon was hand dipped to apply zinc coating, removed from the galvanizing bath, and delayed five seconds before immersion in a room temperature saturated aqueous tin-chloride solution for about 3 seconds. At the t.'me of immersion, the coupon had a temperature of from about 430 to about 440 degrees centigrade. Coating temperature was within the desired range (ZOO-250 C.) for about two seconds. The tin coating had a good appearance, was bright, and no pin holes were observed.

Another coupon was treated in substantially the same manner except that immersion in the tin-chloride solution was delayed for 120 seconds after removal from the galvanizing bath. 0n immersion, the coupon had a temperature of from about 230 to about 240 degrees centigrade and was immersed for about 3 seconds. Coating temperature was within the desired range (ZOO-250 C.) for about two seconds. Inspection of the tin coating revealed a bright, pin-hole free coating.

Example 3.-Weathered zinc-galvanized steel coupons having the dimensions 1 inch by 4 inches by 16 gauge were heated with a natural gas flame to temperatures in the range of from about 300 to about 450 degrees centigrade. They were immediately quenched in a 200 degree centigrade, aqueous tin chloride solution as described in Example 1 for from about 2 to about 4 seconds and removed. Coating temperature was within the desired range (ZOO-250 C.) for about two seconds or more. A typical coupon weighed 17.8836 grams prior to quenching and 18.2162 grams after quenching. The tin coatings obtained were bright, tight, and free of pin holes.

Example 4.Galvanized coupons were immersed in aqueous tin chloride solutions of various concentrations for from about 2 to about 4 seconds. Representative data are tabulated below.

Weight Percent Coupon Percent Solution Temp. on Coating Temp, Immersion, Bright SnCl: H O ZIlClg 0. C. and

Tight 1 0 200 200-250 100 46. 4 53. 6 0 Room 200-250 100 13. 0 87. 0 0 Room 200-250 100 6. 25 94. 0 Room 200-250 6. 25 50. G 43. 15 Room 200-25!) 4. 26 51. 6 44. 14 Room 200-250 70 36. 7 42. 3 21. 1 Room 200-250 100 0 Room 10 0 230 Room 100 1 Remainder dark and spongy. Less than 100% represents an unaceept able coating.

2 SnCl2'2HzO.

Example 5 .-A steel coupon, substantially as described above, was dip-coated with lead in a molten (370 C.)

lead bath, removed from the molten-lead bath, and immediately immersed in a room-temperature, saturated aqueous tin chloride solution. After immersion at coating temperature (200250 C.) for a period of time of about two seconds, the coupon was removed from the aqueous solution.

Inspection of the tin coating thus obtained revealed a lustrous, tight coating which was free of pin holes.

Example 6.A zinc galvanized steel coupon was heated to a temperature of about 430 degrees centigrade. Saturated aqueous stannous chloride solution at room temperature was sprayed onto the coupon so as to thoroughly cover it with solution. Coating tem erature was within the deisred range (200-250 C.) for at least two seconds. The solution was then removed by rinsing.

Observation of the coating of tin thus obtained revealed a tight, pin-hole free coating. Further tests indicated that the coating did not readily react with aqueous HCl.

Various modifications may be made in the present invention without departing from the spirit or scope thereof, and it is to be understood that I limit myself only as defined in the appended claims.

I claim:

1. A method of coating tin over a basis metal selected from the group consisting of lead, zinc, and mixtures and alloys containing major proportions thereof which comprises providing an aqueous solution containing from about ten percent by weight to the saturation limit of a water-soluble tin salt, providing a basis metal to be coated, contacting said basis metal with said aqueous solution for at least two seconds at a temperature of from about 200 to about 250 degrees centigrade, and removing the solution from the tin-coated basis metal.

2. A method of coating tin over a basis metal selected from the group consisting of lead, zinc, and mixtures and alloys containing major proportions thereof which comprises providing an aqueous solution containing from about six percent by weight to the saturation limit of a water-soluble tin salt and at least about percent by weight of zinc chloride, providing a basis metal to be coated, contacting said basis metal with said aqueous solution for at least two seconds at a temperature of from about 200 to about 250 degrees centigrade, and removing the solution from the tin-coated basis metal.

3. A method of coating tin over zinc which comprises providing an aqueous solution containing from about ten percent by weight to the saturation limit of a water-soluble tin salt, contacting said zinc with said aqueous solution for at least two seconds at a temperature of from about 200 to about 250 degrees centigrade, and removing the solution from the tin-coated zinc.

4. A method of coating tin over zinc which comprises providing an aqueous solution containing from about six percent by weight to the saturation limit of a water-soluble tin salt and at least about 20 percent by weight of zinc chloride, contacting said zinc with said aqueous solution for at least two seconds at a temperature of from about 200 to about 250 degrees centigrade, and removing the solution from the tin-coated zinc.

5. A method of coating tin over zinc which comprises heating zinc-galvanized steel to a temperature of from about 300 to about 450 degrees centigrade, quenching said zinc-galvanized steel in an aqueous solution containing about 17 percent by weight stannous chloride, said solution being at a temperature of about 200 degreescentigrade, thereby coating tin over the zinc at a coating temperature of from about 200 to about 250 degrees centigrade, and removing the tin-coated zinc-galvanized steel from the solution after at least about two seconds at the coating temperature.

6. A method of coating tin over zinc which comprises providing zinc-galvanized steel at a temperature of from about 230 to about 470 degrees centigrade, providing an aqueous bath containing about 17 percent by weight stannous chloride, said bath being at room temperature, immersing said zinc-galvanized steel in said bath, thereby coating tin over the zinc at a coating temperature of from about 200 to about 250 degrees centigrade, and removing the tin-coated zinc-galvanized steel from the solution after at least about two seconds at the coating temperature.

7. A method of coating tin over zinc which comprises providing zinc-galvanized steel at a temperature of about 430 degrees centigrade, providing a saturated aqueous stannous chloride solution, spraying said stannous chloride solution on said zinc-galvanized steel in an amount sufficient to cover said steel, thereby coating tin over the zinc at a coating temperature of from about 200 to about 250 degrees centigrade, and removing the solution from the tin-coated zinc.

References Cited UNITED STATES PATENTS 204,297 5/1878 Crooke et al 117-130 1,405,167 1/1922 Shoemaker 117-7l 2,057,762 10/1936 Boone et al 29196.5 X 2,543,365 2/1951 Harris 117l30 X 2,873,214 2/1959 Schnable 1l7130 ALFRED L. LEAVITT, Primary Examiner.

l. R. BATTEN, JR., Assistant Examiner. 

6. A METHOD OF COATING TIN OVER ZINC WHICH COMPRISES PROVIDING ZINC-GALVANIZED STEEL AT A TEMPERATURE OF FROM ABOUT 230 TO ABOUT 470 DEGEES CENTIGRADE, PROVIDING AN AQUEOUS BATH CONTAINING ABOUT 17 PERCENT BY WEIGHT STANNOUS CHLORIDE, SAID BATH BEING AT ROOM TEMPERATURE, IMMERSING SAID ZINC-GALVANIZED STEEL IN SAID BATH, THEREBY COATING TIN OVER THE ZINC AT A COATING TEMPERATURE OF FROM ABOUT 200 TO ABOUT 250 DEGREES CENTIGRADE, AND REMOVING THE TIN-COATED ZINC-GALVANIZED STEEL FROM THE SOLUTION AFTER AT LEAST ABOUT TWO SECONDS AT THE COATING TEMPERATURE. 